Constant rate incremental positioning circuit for crt x-y plotter display



Dec. 31, 1968 L ROTHSCHILD ETAL 3,419,750

CONSTANT RATE INCREMENTAL POSITIONING CIRCUIT FOR CRT X-Y PLOTTERDISPLAY Filed July 28. 1965 /lff F: IE

' DFFL ECT/0N @HHN/VFL Y' D'FL ECT/0A! CHAN/VE L VOL TA6! United StatesPatent Office 3,419,750 CONSTANT RATE INCREMENTAL POSITIONING CIRCUITFOR CRT X-Y PLOTTER DISPLAY Louis I. Rothschild, Elkins Park, Pa., JamesV. VWilliamson, Palm Beach Gardens, Fla., and Michael N. Leemng, NewYork, N.Y., assignors to the United States of America as represented bythe Secretary of the Air Force Filed July 28, 1965, Ser. No. 475,618 1Claim. (Cl. 315-18) ABSTRACT OF THE DISCLOSURE A deflection circuit forbeam writing display tubes to provide a constant writing rate includingan incremental summing means, an amplifier connected to the summingmeans, a damped oscillatory charging circuit for slow beam deflection inthe writing mode and a pair of oppositely polarized diodes in parallelto permit rapid beam deflection when accessing a position from which totrace a line.

This invention relates to a deflection circuit for cathode ray tubes,and more particularly to an incremental positioning circuit whichprovides a constant writing rate for cathode ray display tubes orcomputer read-out tubes.

The use of high speed receivers in modern electronic graphictransmission systems has presented the need for better control ofelectron beam deflection in display devices. This need is acute wherethe display is a large cathode ray tube, in the order of 16 to 19 inchesor more.

The large, high speed cathode ray tubes have, in general, performed wellin data processing applications and specifically for such things as therapid read-out of library data, display of alphanumeric characters andfor geographical displays. One of the major drawbacks of these displayshas been in instances where the tubes are to provide a visual displaywhich is the subject of high resolution photography.

Formerly circuits utilized for controlling the beam deilection in thesetubes increased the voltage to the deflection electrodes sharply. Thissharp rise causes an uneven decay in the light energy given off by thephosphor of the tube, and as a result photographs poorly. Before the newand novel circuit of this invention such abrupt voltage changes werenecessary because of the speed desired and required when changing theinitial writing position of the beam. Prior to this invention there wasno way of deflecting the electron beam at a constant rate for writingand also deflecting the beam rapidly for accessing a new position on theface of the tube.

It is, therefore, an object of this invention to provide a circuit for acathode ray display tube with a constant deflection rate.

It is another object of this invention to provide a cathode ray displaytube with a circuit for constant beam deflection but rapid response forchanges in initial writing position.

Still another object of this invention is to provide a cathode ray tubecircuit with rapid deflection when accessing a position from which totrace a line.

These and other advantages, features and objects of the invention willbecome more apparent from the following description taken in connectionwith the illustrative embodiment in the accompanying drawings, wherein:

FIGURE l is a schematic diagram of the circuit embodied in thisinvention; and

FIGURE 2 is a curve illustrating the increase in voltage relative totime provided by the circuit of this invention.

Referring now to FIGURE 1, there is shown generally 3,419,750y PatentedDec. 31, 1968 a cathode ray display tube 10 with x and y electrostaticdeflection plates 12 and 13, respectively. The x and y deflectionchannels are shown generally at 14 and 16, respectively.

In the x deflection channel, which is similar to the y channel, there isshown a well known D.C. computer summing circuit 18. The currentsflowing through the resistors R1, R2, R3 RN in the D/A (digital-analog)network are summed at N and control voltage VA, the output of amplifierA, in accordance with the following equation:

1 1 1 1 rfi-NRS) (11311511.51R3 RN where VR is the reference voltage andR5 is the feed back resistor.

As the switches (S1 SN) in the D/A network open or closed, the stepfunction of the current flow appears at N and causes correspondingabrupt changes in the voltage VA as shown at 20 in FIGURE 2. Theaddition of the components C1, L1, R4, D1 and D2 change the stepfunction of the voltage developed at VA to an essentially linear ramp asshown at 22 of FIGURE 2, between T1 and T2, the time when the beam isintensified.

The ramp function is the voltage measured across the condenser C1 in thedamped oscillatory charging circuit comprising condenser C1, inductanceL1 and resistor R1.

The voltages developed across the inductance L1 for operation in theincremental beam writing mode are smaller than the amount of voltagerequired to obtain forward resistances in the diodes D1 and D2, whichare lower in turn than the resistance of R4. The voltages across L1 inthe beam writing mode are below the knee of the forward I-Echaracteristic of the diode and in general 0.2 volts.

When accessing a position from which to trace a line, however, largechanges in the voltage VA, are required. Such voltages, in the order of0.5 volt or more, are needed to cause a greater deflection in the beam,and because of the design purpose of the tube, such changes in voltagesmust occur rapidly.

These rapid changes in the circuit are accomplished by the switchingaction of diodes D1 and D2. The forward resistances of the diodes underhigh voltage Vbecome substantially less than the impedance of inductanceL1 and resistance R4 in parallel and hence effectively short circuit theinductor L1.

The output from the x and y deflection channels are fed to the displaytube in la normal manner resulting in a constant writing rate, uniformexcitation of the cathode ray tube phosphor per unit area, and uniformlight energy emission from the phosphor for high resolution photographyon the one hand and rapid changes in the electron beam positioning onthe other.

Although the invention has been described with reference to a particularembodiment, it will be understood to those skilled in the art that theinvention is capable of a variety of alternative embodiments within thespirit and scope of the appended clai-m.

What is claimed is:

1. An electron beam deflection circuit for display tubes comprising: ameans for summing a plurality of currents, consisting of a plurality ofserially connected switch and resistor modules connected in parallel; anamplifier means connected to said summing means; a damped oscillatorycharging circuit, comprising a capacitor connected serially to and aresistor connected parallel to an inductance, connected between theinput to the amplifier and the output of the amplifier; a pair ofnon-linear impedance devices connected in parallel with said inductanceand each 3 4 having its easy path of current flow in the opposite direc-References Cited tion; and output means coupled to the said dampedoscil- UNlTED STATES PATENTS latory charging circuit and connected tothe deflection plates of a display tube, whereby when small voltages are313101705 3/1967 Nlcholson 315-27 applied to the deection circuitchanges in output are 5 gradual and when large voltages are applied tothe circuit RODNEY D' BENNETT Primary Examiner' the output changesrapidly. BRIAN L. RIBANDO, Assistant Examiner.

